Electroluminescent lamp having a transparent nickel, chromium, iron alloy metal electrode



July 4, 1967 J. F. MOTSON 3,329,850

ELECTROLUMINESCENT LAMP HAVING A TRANSPARENT j NICKEL, CHROMIUM, IRON ALLOY METAL ELECTRODE Filed Sept. 10, 1963 INVENTOR. JAMES F. MOTSON V A QBNE United States Patent Gfifice 3,329,850 Patented July 4, 1967 3,329,850 ELECTROLUMINESCENT LAMP HAVING A TRANSPARENT NICKEL, CHROMIUM, IRON ALLOY METAL ELECTRODE James F. Motson, 798 Welsh Road, Huntingdon Valley, Pa. 19006 Filed Sept. 10, 1963, Ser. No. 307,797 4 Claims. (Cl. 313108) This invention relates to electroluminescent sources of light.

The fabrication of electroluminescent lamps is well known and each of the fabrication techniques is basically similar. Each technique provides for encapsulating electroluminescent phosphors (such as zinc sulphide), with suitable activators (such as copper sulphide, copper oxide, manganese particles and the like), in a layer of electrical non-conducting material. Each technique provides for disposing the above described layer of electroluminescent material between two electrodes, one of which is transparent, and further provides for applying an alternating electrostatic field across the electroluminescent layer. Thus far a common problem to all electroluminescent lamps fabricated strictly according to above described technique is that the output of light from the electroluminescent layer is not satisfactory.

Many innovations of the basic technique, described above, have been introduced in order to provide an improved light output. It 'has been generally accepted that if either the voltage or the frequency (or both) of the applied signal is increased, the light output will increase. The increase of the voltage and frequency of the applied signal is limited at the point where the dielectric material holding the phosphors breaks down and permits a short circuit commonly called sparking. Methods have been introduced which provide for employing encapsulating material which is of a higher dielectric constant than the electroluminescent phosphors thereby allowing for increased voltage and frequency of the applied signal with some reduction in sparking. Other methods have suggested heating the lamp package by applying a direct current (DC) signal to one of the electrodes or to an auxiliary heating element, in addition to the alternating current (AC) signal, for improved light output.

In addition to the general acceptance of improved performance in response to increased applied'voltage and frequency, it has also been generally accepted that the greater the voltage drop developed directly across the electroluuminescent layer, the greater the light output. According to this last described theory some of the techniques have provided for using a photoconductive layer whose resistance lowers in response to the light from the electroluminescent phosphors and hence provides for the major voltage drop to be across the electroluminescent layer, per se. In general, the techniques have all provided for low resistance electrodes to effect the major voltage drop across the electroluminescent layer, per se.

Despite all the foregoing techniques the quest for improved electroluminescent light output has continued because for many applications such light output is as yet considered unsatisfactory.

Accordingly it is an object of my invention to provide an electroluminescent lamp with an improved light output.

It is a further object of the present invention to provide an improved transparent electrode to be used with an electroluminescent lamp.

In accordance with a feature of the present invention the transparent electrode or both electrodes of my lamp are made of relatively high resistance material but of thin physical dimensions.

In accordance with another feature of my present invention said high resistance material has good stability and can be worked to form a solid sheet in thickness of between 50 angstroms to 150 angstroms.

In accordance with another feature of my invention said high resistance material is colorless when applied in layers of 50 to 150 angstroms.

The figure is a schematic of an electroluminescent lamp wherein layer 1 is the back electrode; layer 2 is the layer of material (such as epoxy resin) which houses the electroluminescent phosphers; layer 3 is a layer of glass, and layer 4 is a transparent electorde of anti-static material (preferably Inconel) which is between 50 to 150 angstroms thick. The electrodes 1 and 4 are shown connected to an AC. source of signal 5. While the electrode 4 is shown bonded to glass it can be evaporated directly on the epoxy resin of layer 2.

The method of fabrication which is described in my U.S. Patent No. 3,037,138 has thus far proved to be an excellent method for improving the light output. The

present invention will be described in connection with the technique of my U.S. Patent 3,037,138 and as an improvement thereof, although the present invention can be used with any electroluminescent lamp having a transparent electrode.

I have found that if the transparent electrode is made by coating a layer of anti-static metal, such as a compound of nickel, 14% chromium and 6% iron (which is commercially sold as Inconel), where the thickness is between 50 angstroms to 150 angstroms and the resistance of the material is between 1,000 ohms per square to 50,000 ohms per square, the electroluminescent lamp provides an improved light output.

Heretofore the transparent electrodes have been made of gold, stannous chloride, tin chloride and the like, which materials have been vapor deposited on a glass base. The foregoing metals are all considered good electrical conductors whose resistance is in the range of 20 ohms per square to 150 ohms per square for a thickness of approximately angstroms. At such a thickness gold, for instance, transmits only 70% of the light incident thereto.

In the preferred embodiment of my present invention I provide an evaporated coating of between 50 angstroms to angstroms of the anti-static metal (Inconel) made up of nickel, chromium and iron on my transparent electrode base. Inconel in such thickness permits 75% to 90% of the light incident thereto to be transmitted therethrough. Further Inconel is colorless at the above thickness whereas other metals contaminate the light output by adding their color characteristic.

It should be understood that other materials such as alloys of chromium might be used provided that they can be worked to form a solid sheet of metal at thicknesses between 50 angstroms and 150 angstroms; have a colorless characteristic at these thicknesses; have good stability; have a resistance value of between 1,000 to 50,000 ohms per square; and of course provided that they transmit at least 75 of the light incident thereto. Throughout I refer to anti-static metals and mean thereby the family of metals normally coated on plastic material to conduct off the static charges that develop thereon.

As indicated earlier it was believed heretofore that the greater the voltage drop developed across the electroluminescent layer per se, the greater would be the light output. Contrary to this belief I have found that increased resistance in the transparent electrode apparently does not reduce the effective electrostatic field acting on the phosphors or if it does it reduces it an insignificant amount because for the same applied voltage and frequency, in

the lower signal range, I have observed virtually the same light output between lamps having a gold deposited transparent electrode and a lamp having an Inconel deposited transparent electrode.

However, while the two lamps provide virtually the same light output at a lower applied voltage and frequency, if I employ Inconel I have a means for improving the light output. There is usually a physical limitation on the size of the lamp package since these lamps are so often employed in instrument panels. The advantage of the use of Inconel is emphasized because of this limitation on package size. Since Inconel has the capacity to transmit a greater percentage of the light incident thereto than does gold, tin chloride and the like, it follows that for the same light generated, a greater percentage of light will be seen as coming from the lamp. Further since Inconel has a greater resistance than gold, tin chloride and the like, it is possible to apply signals of greater voltages and frequencies without causing a short circuit or sparking and this feature is extremely important. My invention does not contradict the basic concept; the greater the applied voltage, the greater will be the light output. The fact that greater applied voltages and frequencies can be applied has been found to be especially useful in adapting the present lamp to use in aircraft whereat the power supply can be 115-300 volts and 4001600 cycles and wherein as stated before the light package must be thin. An advantage also lies in the fact that Inconel is colorless for thicknesses between 50 angstroms to 150 angstroms. Further the increased resistance in the electrodes, or electrode, coupled with the increased applied voltage provides for a greater 1 R factor across the lamp and hence a better light output. According to Destriaus treatise New Phenomenon of Electrophotoluminescence, Philosophical Magazine, October 1947, the brightness of an elecrtoluminescent lamp is increased with the increase in absolute temperature of the cell. Destriau explains that the threshold of visible luminescence for an electroluminescent cell is lower at a higher temperature. I believe in view of the foregoing theory that as the applied power increases, the I R factor increases and helps improve the lamp output, such improvement being in addition to the improvement resulting from the increased electrostatic field developed across the lamp.

The increased resistance in the transparent electrode could be effected in the non-transparent electrode or in both electrodes but the implementation of such a procedure has not been found efficient. There is a limit beyond which the increased resistance has a no appreciable effect for betterment of the lamp and this limitation appears to be about 50,000 ohms per square. Since the metal on the transparent electrode is made thin in order to permit light transmission therethrough, the resistance of such a thin layer of metal is inherently high. The adaptation of the necessary thickness requisites to the metal of the transparent layer in order to provide the proper resistances is all that is necessary to produce the improved lamp as desired.

When Inconel is employed in a thickness of between 50 angstroms and 150 angstroms to give a 75% to 90% light transmission, it is in the form of a continuous layer or sheet which is highly desirable for stability. That is to say Inconel at the presently descrbed thicknesses shows a constant resistance value over a long life and through severe environmental cycling. By contrast when gold, and the like, is employed to give its maximum light transmission, a 70% light transmission, the gold is observed to be in granular'form which is undesirable for stability. Gold must be built up to a thickness which will provide 21' only 65% transmission in order for it to be a continuous sheet.

In summary then I have found that if a transparent electrode to be used in an electroluminescent lamp is fabricated by evaporation deposition of an anti-static metal, having the characteristics of Inconel, to a thickness of between 50 angstroms and 150 angstroms, directly on the transparent dielectric (for instance epoxy resin) holding the phosphors, or as an entity on a glass base or a plastic base, wherein the resistance of said anti-static metal is in the range of 1,000 ohms per square to 50,000 ohms per square, and such transparent electrode is employed in an electroluminescent lamp (of the type described in my US. Patent 3,037,138) I can provide a lamp which is capable of producing an increased light output.

This invention is to be considered as including an electroluminescent lamp employing the above described transparent electrode as well as the transparent electrode per se. In the claims the term electroluminescent layer is to be considered as a plurality of zinc sulphide phosphors (or similar electroluminescent phosphors) and a plurality of copper sulphide activators (or similar activators) encapsulated in a plastic material such as epoxy resin.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

\Vhat is claimed is:

1. A transparent electrode to be used with an electroluminescent lamp comprising: a transparent base means having two relatively large surfaces disposed opposite each other; a layer of metal composed of approximately 80% nickel, 14% chromium and 6% iron which has been evaporated-coated on one of said large surfaces of said transparent base means in a thickness of between 50 angstroms and 150 angstroms.

2. A transparent electrode device according to claim 1 wherein said transparent base is a plastic base.

3. A transparent electrode device according to claim 1 wherein the anti-stati metal will work into a solid sheet of metal at said thickness of 50 angstroms to 150 angstroms and wherein said anti-static metal is colorless at said last-mentioned thicknesses.

4. An electroluminescent light source comprising: a first electrode; an electroluminescent layer bonded to said first electrode; a second electrode including a layer of transparent metal alloy coated thereon in a thickness of between 50 angstroms and 150 angstroms, said transparent metal alloy being made up of approximately 80% nickel, 14% chromium and 6% iron; said second electrode bonded to the other side of said electroluminescent layer; and means coupled to said first and second electrodes to be connected to an alternating current power JAMES W. LAWRENCE, Primaly Examiner.

DAVID J. GALVIN, Examiner.

R. JUDD, Assistant Examiner. 

1. A TRANSPARENT ELECTRODE TO BE USED WITH AN ELECTROLUMINESCENT LAMP COMPRISING: A TRANSPARENT BASE MEANS HAVING TWO RELATIVELY LARGE SURFACES DISPOSED OPPOSITE EACH OTHER; A LAYER OF METAL COMPOSED OF APPROXIMATELY 80% NICKEL, 14% CHROMIUM AND 6% IRON WHICH HAS BEEN EVAPORATED-COATED ON ONE OF SAID LARGE SURFACES OF SAID TRANSPARENT BASE MEANS IN A THICKNESS OF BETWEEN 50 ANGESTROMS AND 150 ANGSTROMS. 